A photoplethysmogram (PPG) signal is commonly utilized to monitor physiological metrics of an individual, such as cardiac information, for example, a heartbeat signal or a heart-rate. Devices that generate a PPG signal using optical techniques typically include an emitter configured to output light toward skin of the user and a receiver, which may include a photodiode and electrical circuitry, configured to generate a light intensity signal based on a light reflections received by the photodiode and using the electrical circuitry to generate a PPG signal based on the light intensity signal. The PPG signal typically includes a cardiac component, a motion component, a respiratory component and other components may be included as well.
To effectively monitor the physiological characteristic, several factors that may adversely affect the measurement of the physiological characteristic are generally considered. For example, a PPG signal quality for a desired cardiac component of the PPG signal is sensitive to variations in operating parameters of the device and biometric characteristics of the user (e.g., skin tone or complexion, skin density, body fat of the user, moisture level of the user's skin, hair density or color, etc.) at a location on the user's body (e.g., the user's wrist) against which the monitoring device is positioned to output light and receive reflections (e.g., output to and received from the skin on the top of the user's wrist). Operating parameters of the device may include a particular gain setting, number of channels for one or more photodiodes, a number of pulses of light output by one or more emitters (LEDs), a width of each pulse output by the one or more emitters (LEDs), a voltage of the one or more emitters (LEDs), or a current of the one or more emitters (LEDs). These variations may rapidly degrade the signal-to-noise ratio (SNR) for the cardiac component of the PPG signal and adversely impact the accuracy of measurements. Additionally, the components utilized to perform the measurements have an inherent range of operating characteristics that may affect the accuracy of the monitoring device.
Known techniques for accounting for certain variations of amongst users of fitness monitors includes characterizing the user by determining mathematical relationships for each user while the device is in use. For example, a fitness monitor may operate in a first mode to measure a heart rate by using light pulses of constant intensity and a characterization mode to determine a relationship between varying intensity levels of light and collected data points. For instance, the characterization mode may include varying the intensity of a light source, measuring light reflected to a photodiode and determining a relationship between the collected data points and the varying levels of light. It is therefore desired to provide a monitoring device, for example, a fitness monitor device, that characterizes the fitness monitor instead of characterizing the user while the device is in use. Characterization of a fitness monitor based on its operation enables the fitness monitor to account for variations in operating parameters of the device and biometric characteristics of the user without utilizing a characterization mode.